The present invention relates to a filter comprising a first end, a second end, a conducting casing, at least one central conductor, and one connecting means at the ends, respectively. The casing extends along an imaginary axis and is intended to accommodate the conductor along this imaginary axis, and further the conductor is provided with alternating capacitance generating portions and inductance generating portions. The connecting means at the first end of the filter comprises a first connecting member connected to the casing and a second connecting member connected to the conductor, and the connecting means at the second end of the filter comprises a third connecting member connected to the casing and a fourth connecting member connected to the conductor.
Such a filter is known from JP-08018305 A, which accordingly describes the technique, which is used as a base for the invention.
In radio base stations, frequency filters, especially low-pass filters (LP-filters), are used. The filters comprises separate units and can be integrated with and connected to other units by means of soldering or mechanical joining. Connections involve always a risk for, for instance, bad electrical contact and the more pieces and connections there are the more problems with contact will arise. Another problem with conventional LP-filters, where the conductor is enclosed in a plastic tube, which in turn is enclosed in a conducting casing, is that there could be a play between the insulator and the two conductors of the capacitor, which will give rise to a bad defined capacitance, which means an uncertain interval for the passed frequency.
The above mentioned problems are solved by a filter of the above defined kind, which is characterised in that both the second and fourth connecting members are provided integrally with the conductor. The finesse is that the filter is integrated with a connector. The contact is actually the end of the conductor of the filter.
It is suitable that an insulator covers at least one part of at least one of the capacitance generating portions of the conductor and that the insulator is arranged on said portion. The insulator gives a protection against breakdown voltage across the capacitance generating portions and the casing.
Preferably the insulator is cast directly on said capacitance generating portions. This provides that the capacitance can be determined more precisely since play that can occur between the insulator and one of the conductors of the capacitor is avoided.
Preferably there is at least one axially directed junction means between each capacitance generating portion, which connects the insulator around these with each other, the junction means consisting of the same material that is cast around the portions, the insulator and the junction means providing a continuous unit. The junction means thereby enables the casting of the insulator in one piece.
Suitably the casing is provided with at least one stopping means to fasten the conductor co-axially in the casing.
Further it is advantageous that one of the stopping means is a projection on the inside of the casing at one of the ends, whereby the capacitance generating portion, which is closest to the second connecting member and together with the insulator has a diameter that is sized such that it precisely fits in the rest of the casing, is stopped at the projection.
It is suitable that at least one of the stopping means comprises at least one emboss in the casing, this emboss causing a decrease in the inner diameter of the casing. This decrease in diameter pushes preferably the capacitance generating portion that is closest to the second or fourth connecting member, and thus the whole conductor, axially towards the stopping means in the opposite end.
Further at least one of the stopping means can comprise a ring of insulating material, which is placed around the soldering connection end against one part of the conductor, which has a slightly larger diameter than the soldering connection end and a ring-shaped edge, outermost at the other end of the casing, which is folded in over the ring, this folding causing that the ring and thereby the whole conductor is pressed against the stopping means in the opposite end. This contributes to a mounting of the conductor in the casing without play.
At least one of the second and fourth connecting members could be a soldering connection end.
It is suitable that at least one of the second and fourth connecting members is provided as a circularly cylindrical shell extended in the longitudinal direction, one of the ends of which is free, and from this is at least one axially directed slit taken from the free end towards the capacitance generating portion that is closest to the fourth connecting member.
Preferably the first connecting member and the third connecting member are provided integrally with the casing and the casing is provided in one piece. This provides that grounding problems are decreased since the connector and the filter have the same potential, when they are provided integrally.
Preferably the first and third connecting members are provided with threads. Hereby a safe connection is obtained between the filter and connecting parts.
Preferably the conductor is provided in one piece, whereby the filter is a low-pass filter.
Alternatively, at least one of the capacitance generating portions is divided into two smaller portions, which are following each other in the direction of the axis, the portions being separated by a gap, which is filled with at least one dielectric, for example air, ceramic or a polymeric material, whereby the filter is a band-pass filter.
It is advantageous in the manufacturing that said dielectric is of the same insulating material that is cast around the portions, whereby the insulator and the dielectric together compose a continues unity.
It is further advantageous that the casing as well as the different portions of the conductor are essentially circularly cylindrical, the capacitance generating portions having the largest diameter, and that at least one of the portions is arranged co-axially along the imaginary axis (I).
Suitably, at least the capacitance generating portions and the inductance generating portions of the conductor are silvered. Hereby a good conductivity is obtained since the current is conducted along the surface.
Since the number of pieces is reduced also the manufacturing costs are reduced.